Multilayered polymer films comprising one or more first birefringent optical layers alternated with one or more second layers are known.
As described for example in Hebrink et al. U.S. Pat. No. 6,641,900, polyethylene naphthalate (PEN) is an example of a useful material for forming the first optical layer because it is highly birefringent after stretching. Examples of suitable materials for the second optical layers are copolymers of PEN, PBN, PET, or PBT. One example of a polyester for use in second optical layers is coPEN having carboxylate subunits which are about 70 mol % naphthalate and about 30 mol % isophthalate or terephthalate subunits. Another example is a coPEN wherein 20 to 60 mol % of the carboxylate subunits are naphthalate, 20 to 50 mol % are terephthalate, and 0 to 10 mol % are isophthalate subunits.
Other polymeric materials have been described for use in multilayered optical films. For example, Allen et al., U.S. Pat. No. 6,673,275 describes an optical film wherein at least one of the continuous and disperse phases comprises a blend of homopolymers that are inter-reacted, as by transesterification. The resulting films can exhibit a higher degree of birefringence for a given level of strain than analogous films in which the blend is replace by a random copolymer.